The present invention relates to a noise reduction system for a passenger compartment of an automotive vehicle by positively generating a sound from a sound source to cancel the vehicle internal noise.
There have been proposed several techniques for reducing the noise sound in the passenger compartment by producing a canceling sound, having the same amplitude as the noise sound and a reversed phase thereto, from a sound source disposed in the passenger compartment.
In a recent example, Japanese application laid open No. 1991-178845 disc loses a vehicle internal noise reduction technique for reducing a noise sound by using a LMS (Least Means Square) algorithm (a theory for obtaining a filter coefficient by approximating it to an instanteneous means square error in order to simplify a formula, utilizing that a filter correction formula is a recursive expression or by employing a MEFX-LMS (Multiple Error Filtered X-LMS) algorithm. This technique has already been put to a practical use in some of production vehicles.
Commonly, an internal noise reduction system using this LMS algorithm is composed such a way that: a vibration noise source signal (primary source) is detected from an engine, then the primary source is synthesized with a filter coefficient of an adaptive filter into a canceling sound, then the canceling sound is generated from a speaker to cancel a noise sound in the passenger compartment, further the noise sound reduced by the canceling sound is detected as an error signal by a microphone disposed at a noise receiving point, and based on the detected error signal and a primary source signal synthesized with a compensation coefficient (a coefficient mainly representing a speaker/microphone transmission characteristic as a finite impulse response), a filter coefficient of the adaptive filter is updated by the LMS algorithm so as to optimize the reduced noise sound at the noise receiving point.
However, in the above mentioned internal noise reduction system utilizing a LMS algorithm or a MEFX-LMS algorithm, there is a problem that the compensation coefficient representing a speaker/microphone characteristic deviates substantially from a desired value when the speaker/microphone characteristic varies due to changes of miscellaneous conditions within the passenger compartment, such as a change of the number of passengers, a change of temperature in the passenger compartment and a deterioration of the speaker performances, and consequently a deviation of time, namely a deviation of phase is caused and a result it becomes difficult to reduce a noise because of the erroneous positioning where an adaptive filter is to be updated.